# std::includes

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt1, class InputIt2 > bool includes( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > bool includes( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2 ); | (2) | (since C++17)
template< class InputIt1, class InputIt2, class Compare > bool includes( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, Compare comp ); | (3) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class Compare > bool includes( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2, Compare comp ); | (4) | (since C++17)

Returns true if the sorted range `[`first2`,` last2`)` is a subsequence of the sorted range `[`first1`,` last1`)` (a subsequence need not be contiguous).

1) If `[`first1`,` last1`)` or `[`first2`,` last2`)` is not sorted with respect to operator<(until C++20)std::less{}(since C++20), the behavior is undefined.
3) If `[`first1`,` last1`)` or `[`first2`,` last2`)` is not sorted with respect to comp, the behavior is undefined.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### Parameters

first1, last1 | - | the sorted range of elements to examine
first2, last2 | - | the sorted range of elements to search for
policy | - | the execution policy to use
comp | - | comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1& a, const Type2& b); While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) `Type1` and `Type2` regardless of value category (thus, `Type1&` is not allowed, nor is `Type1` unless for `Type1` a move is equivalent to a copy(since C++11)). The types Type1 and Type2 must be such that an object of type InputIt can be dereferenced and then implicitly converted to both of them. ​
Type requirements
- `InputIt1, InputIt2` must meet the requirements of LegacyInputIterator.
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator.
- `Compare` must meet the requirements of Compare.

### Return value

true if `[`first2`,` last2`)` is a subsequence of `[`first1`,` last1`)`; otherwise false.

An empty sequence is a subsequence of any sequence, so true is returned if `[`first2`,` last2`)` is empty.

### Complexity

Given \(\scriptsize N_1\)N1 as std::distance(first1, last1) and \(\scriptsize N_2\)N2 as std::distance(first2, last2):

1,2) At most \(\scriptsize 2 \cdot (N_1+N_2)-1\)2⋅(N1+N2)-1 comparisons using operator<(until C++20)std::less{}(since C++20).
3,4) At most \(\scriptsize 2 \cdot (N_1+N_2)-1\)2⋅(N1+N2)-1 applications of the comparison function comp.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

include (1)
    template<class InputIt1, class InputIt2>
    bool includes(InputIt1 first1, InputIt1 last1,
                  InputIt2 first2, InputIt2 last2)
    {
        for (; first2 != last2; ++first1)
        {
            if (first1 == last1 || *first2 < *first1)
                return false;
            if (!(*first1 < *first2))
                ++first2;
        }
        return true;
    }
include (3)
    template<class InputIt1, class InputIt2, class Compare>
    bool includes(InputIt1 first1, InputIt1 last1,
                  InputIt2 first2, InputIt2 last2, Compare comp)
    {
        for (; first2 != last2; ++first1)
        {
            if (first1 == last1 || comp(*first2, *first1))
                return false;
            if (!comp(*first1, *first2))
                ++first2;
        }
        return true;
    }

### Example

Run this code

    #include <algorithm>
    #include <cctype>
    #include <iostream>

    template<class Os, class Co>
    Os& operator<<(Os& os, const Co& v)
    {
        for (const auto& i : v)
            os << i << ' ';
        return os << '\t';
    }

    int main()
    {
        const auto
            v1 = {'a', 'b', 'c', 'f', 'h', 'x'},
            v2 = {'a', 'b', 'c'},
            v3 = {'a', 'c'},
            v4 = {'a', 'a', 'b'},
            v5 = {'g'},
            v6 = {'a', 'c', 'g'},
            v7 = {'A', 'B', 'C'};

        auto no_case = [](char a, char b) { return std::tolower(a) < std::tolower(b); };

        std::cout
        << v1 << "\nincludes:\n" << std::boolalpha
        << v2 << ": " << std::includes(v1.begin(), v1.end(), v2.begin(), v2.end()) << '\n'
        << v3 << ": " << std::includes(v1.begin(), v1.end(), v3.begin(), v3.end()) << '\n'
        << v4 << ": " << std::includes(v1.begin(), v1.end(), v4.begin(), v4.end()) << '\n'
        << v5 << ": " << std::includes(v1.begin(), v1.end(), v5.begin(), v5.end()) << '\n'
        << v6 << ": " << std::includes(v1.begin(), v1.end(), v6.begin(), v6.end()) << '\n'
        << v7 << ": " << std::includes(v1.begin(), v1.end(), v7.begin(), v7.end(), no_case)
              << " (case-insensitive)\n";
    }

Output:

    a b c f h x
    includes:
    a b c   : true
    a c     : true
    a a b   : false
    g       : false
    a c g   : false
    A B C   : true (case-insensitive)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 1205 | C++98 | the return value was unclear if `[`first2`,` last2`)` is empty | returns true in this case

### See also

set_difference | computes the difference between two sets (function template)
search | searches for the first occurrence of a range of elements (function template)
ranges::includes (C++20) | returns true if one sequence is a subsequence of another (algorithm function object)

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